Crosslinked cellulose fiber has excellent absorbent properties and therefore has been used in the manufacture of products such as diapers and absorbent towels. Often, the fibers undergo crosslinking at a site remote from the product manufacturer's location. It is therefore common to transport the crosslinked fiber from its production site to the manufacturer's location. However, due to their bulk, transportation of such fibers can be costly.
Also, if such fibers are transported in bags or bales, they tend to form clumps during transport. An example of baling high-bulk fiber for transport is set forth in U.S. patent application Ser. No. 07/607,265, entitled "Method for Packaging and Shipping Fiber Material" filed Oct. 31, 1990, now U.S. Pat. No. 5,174,198. This case discloses restraining bales of crosslinked fibers by wires to prevent the fibers from expanding during transportation. One drawback of this approach is that refiberation equipment is typically required at the product manufacturer's facility to break up the agglomerated fibers prior to air laying or otherwise incorporating them into products.
For applications as absorbent products, it is desirable that compressed fibers retain their absorbency once the compression force is released. Balancing the need to compress the crosslinked fibers to facilitate transportation while maintaining the absorbency and springback of the fibers has proven difficult and costly.
Consideration has also been given to forming crosslinked fibers into undensified web structures and then transporting the preformed undensified webs, as in rolls, to the product manufacturer's facility. This eliminates the need to refiberize the webs if the webs are incorporated directly into a product, such as the core of a diaper. However, in undensified web format the webs are still bulky and the products made from such webs can also be bulky.
Attempts have also been made to compress webs of crosslinked fibers prior to shipment. However, because of the resilience of such fibers, they tend to spring back to their original bulk prior to shipment or prior to incorporation into a manufacturer's product. If enough force is applied to prevent springback, such as by using a hydraulic press, the fibers can be crushed with the absorbent capacity of the webs consequently being severely hampered.
U.S. Pat. No. 4,822,453 recognizes the difficulty of forming densified sheets of crosslinked fibers. One reason for this difficulty is that the fibers substantially expand once the compression force is released. This reference discloses crosslinked fibers loaded with from 0.5 to 3.5 molar percent crosslinking agent. This references also indicates that compressed webs having densities of from 0.12 to 0.6 g/cc have been achieved using a hydraulic press without listing the loading of these webs. However, the absorbent capacities of the webs (measured as the wet densities) for webs compressed to densities of 0.12 to 0.4 g/cc was stated as 0.06 g/cc and 0.12 g/cc. However, a wet density of 0.12 g/cc corresponds to relatively little expansion upon wetting at a relatively low compressed density of 0.40 g/cc. This reference mentions the possibility of increasing the compressibility of the webs by reducing the loading level of crosslinking agent or by mixing crosslinking fibers with uncrosslinked fibers. However, the capacity of webs to absorb liquid is believed to be hampered by reducing the amount of crosslinking agent used or by mixing a blend of crosslinked and uncrosslinked fibers.
Finally, it is known to compress wood pulp under heat and pressure, e.g. in paper making processes. However, compressed wood pulp absorbs relatively little liquid following compression and subsequent rewetting in comparison to the absorbent capacity of the crosslinked fibers.
Therefore, a need exists for improved densified webs of crosslinked fibers and an improved method of making such webs.
Accordingly, a first object of the present invention is to provide an improved method of compressing crosslinked high-bulk fibers into densified absorbent structures.
A second object of the present invention is to provide an improved method of compressing crosslinked fibers which is versatile and which does not cause structural damage to the fibers upon application of the compression force.
A third object of the present invention is to provide highly densified absorbent structures of high-bulk crosslinked fibers which retain their shape upon release of the compression force and yet maintain an excellent absorbent capacity upon wetting.